Hi Glen, A 6" Saxon, Skywatcher or GS Dobsonian is a great choice. If you can stretch the budget to an 8" Dobsonian, then even better, but don't stress about it. And don't worry about a lot of talk you will hear and read about mirror quality. Discussions on telescope mirrors are like audiophiles discussing which speaker cable is better for a surround sound system. 99% of amateur astronomers wouldn't know the difference if their mirror was from a Chinese/Taiwanese production line or handmade by an mirror maker. And that's fine because you just want your son (and you) to enjoy the hobby.
I would tend to look at the latest GSO dobs only because I like the improved altitude bearings (that's bearings on each side of the tube!) Saxon or Skywatcher may have caught up and improved theirs also.
Andrews Communications have them with some good accessories which Eric mentioned, but you may also have a budget.
Don't go any smaller than a 6" however.
See if you can pick up Astronomy 2010 - the yearbook at Andrews,
BinTel, or
Quasar Publishing It's about $20.
Best of luck,
Stephen